The broad, long-term objectives of this proposal are to understand the structure and function of a novel tetraspanin called SARCOSPAN. Sarcospan is an integral component of the dystrophin-glycoprotein complex and is highly expressed in skeletal and cardiac muscles, as well as many non-muscle tissues (Crosbie et al., 1997; Crosbie et al., 1998; Crosbie et al., 1999). The dystrophin-glycoprotein complex (DGC) is a structural complex that spans the muscle plasma membrane and links the extracellular matrix with the intracellular cytoskeleton. This structural linkage is critical for normal muscle function as clearly demonstrated by the many forms of muscular dystrophy that result from mutations in the dystrophin-glycoprotein complex. Association of several signaling molecules with the DGC also suggests that this complex may play a role in mediating extracellular-intracellular communications. Furthermore, lateral associations amongst membrane components of the DGC are critical for function of this complex. It is hypothesized that sarcospan facilitates protein-protein interactions within the dystrophin-glycoprotein complex. These protein interactions are clearly important for the physical linkage between the extracellular matrix and the intracellular actin network and for the prevention of muscular dystrophy. Human mutations within the sarcospan gene have not been identified in known cases of autosomal recessive muscular dystrophy (Crosbie et al., 2000). However, these mutation searches have only examined the ubiquitous form of SSPN, which has a broad expression pattern. Preliminary data demonstrates that a novel, muscle-specific form of SSPN is expressed in skeletal and cardiac muscles. We hypothesize that mutations within muscle-SSPN may cause novel forms of muscular dystrophy. Identification and characterization of this muscle-sarcospan will advance our understanding of the role of the dystrophin-glycoprotein complex in normal muscle and in the pathogenesis of muscular dystrophy.